[fjtorres]

First, Kindle Unlimited isn't the only way to get content on a Kindle ereader or app.
It is entirely possible to get a Kindle reader or app, load it with thousands of ebooks, and never pay Amazon a cent to load the content. You don't even need an app.

You can't say that about Apple, where all content has to go via Apple controlled channels.

So, not a valid comparison.

Second, neither KU nor Amazon are public utilities or mandated in any way.
You don't have to use either.
Amazon doesn't send made men to force you to buy from them nor are they only source of the products they offer. If you are a consumer you can buy from somebody else. If you are a publisher, you are free to go to use any channel for sales. KDP does not demand exclusivity.

Third, Amazon is a for profit business offering a variety of services under a variety of terms. They aren't a government service or charity. They operate under mutually agreeable contracts: they indicate under what conditions they will offer their services. Sometimes they dicker, sometimes they don't. It varies. Their baseline position for KU is exclusivity but they will waive the requirement if it is to their advantage (read: profitable).

Do consider that when it comes to KU and publishers, Amazon is the paying customer, not the subscriber. They are licensing the content and offering it to their subscribers for free. Like Netflix. Like Hulu. Like HBO. Like Disney+. It's their money and they can spend it any way they want. Likewise, providers are free to license their content to KU, three months at a time, or not. Nobody gets shot for walking away from Amazon's money.

There is a large body of law that guarantees businesses, big and small, the right to design their products as they see fit. They do not lose those rights because they are "too successful" in the eyes of some. Likewise, there is no law that says they have to do business with everybody regardless of proposed terms. There is no compulsory licensing or must carry involved.

Businesses have the same right to say "no" as consumers.
Again, that right doesn't go away when the company is successful.

Don't like tbe terms?
Don't do business with them.
Lots of publishers choose not to do business with KU.
Likewise, there are many publishers Amazon chooses not to do business with.
Both are using their legal rights as allowed by law.

Regardless of whoever feels they shouldn't.